In recent years, a server which provides a game to portable devices via a communication network has spread. The game provided by the server includes one in which a plurality of users can participate (a so-called “social game”), in which the users can not only battle and cooperate with each other but also communicate with each other.
As an example of such a social game, Patent Gazette No. JP5118261 describes a game in which a user battles with an enemy character by using a card (a so-called “card battle game”). In the card battle game, a user battles with an enemy character prepared on a game program by using a card while cooperating with another user. In such a game, a user can acquire an item by clearing a mission generated in the game, and can strengthen his/her card by using the item.